It is known that exposure of human or animal tissue to ionising radiation will kill the cells thus exposed. This finds application in the treatment of pathological cells, for example. In order to treat tumours deep within the body of the patient, the radiation must however penetrate the healthy tissue in order to irradiate and destroy the pathological cells. In conventional radiation therapy, large volumes of healthy tissue can thus be exposed to harmful doses of radiation, resulting in prolonged recovery periods for the patient. It is, therefore, desirable to design a device for treating a patient with ionising radiation and treatment protocols so as to expose the pathological tissue to a dose of radiation which will result in the death of those cells, whilst keeping the exposure of healthy tissue to a minimum.
Several methods have previously been employed to achieve the desired pathological cell-destroying exposure whilst keeping the exposure of healthy cells to a minimum. Many methods work by directing radiation at a tumour from a number of directions, either simultaneously from multiple sources or multiple exposures from a single source. The intensity of radiation emanating from each direction is therefore less than would be required to actually destroy cells (although still sufficient to damage the cells), but where the radiation beams from the multiple directions converge, the intensity of radiation is sufficient to deliver a therapeutic dose. By providing radiation from multiple directions, the amount of radiation delivered to surrounding healthy cells can be minimized.
The shape of the beam varies. For single-source devices, cone beams centred on the isocentre are common, while fan beams are also employed (for example as shown in U.S. Pat. No. 5,317,616). Both types of beam require collimation apparatus, in order to shape the beam as required and reduce the irradiation of healthy tissue. There are currently two main types of variable-shape collimator which are in use in conventional radiotherapy machines.
The first, more common type of multi-leaf collimator (“MLC”) has a number of leaves that can be positioned in substantially continuously variable positions (see FIG. 8). The maximum field size is typically square, and the MLC can be rotated so as to orient the leaf direction optimally with regard to the shape of features on the target. Typically, the patient position is adjusted so that the target is at the centre of the MLC, and thus a high resolution MLC can also be of a small size and positioned centrally. Alternatively, composite resolution collimators have been produced where high resolution (i.e. relatively thin) leaves are provided in the central portion and regular width (i.e. relatively thick) leaves are provided in the outer portions. A lower limit to the thickness of the leaves is imposed by the possibility that they may bend under their own weight. No support can be provided, as this would be incompatible with the field defining lamp that is usually provided, and the alternative electron beam that is offered by most linacs.
Fan beam-based systems typically use a binary collimator; this is shaped as a narrow slit, and has a number of leaves positioned along the slit that are either fully closed or fully open (see FIG. 9). The shape of the field is not adjusted, but the time for which the leaves are opened is varied, thereby controlling the radiation fluence that passes though the slit. Due to the slit nature of the collimator, this is used in conjunction with longitudinal motion of the patient so as to cover the extent of the target transverse to the slit.